Feature for probing functions

Introduces a single-flight call to a function's health
endpoint to verify that it is registered with an Istio
sidecar (Envoy) before letting the invocation through.

Results are cached for 5 seconds, before a probe is
required again.

Tested without Istio, with probe_functions environment
variable set to true, I saw a probe execute in the logs.

Fixes: #1721 for Istio users.

Signed-off-by: Alex Ellis (OpenFaaS Ltd) <alex@openfaas.com>

gateway/pkg/middleware/service_name.go

@@ -0,0 +1,10 @@
+package middleware
+
+import "strings"
+
+func GetNamespace(defaultNamespace, fullName string) (string, string) {
+	if index := strings.LastIndex(fullName, "."); index > -1 {
+		return fullName[:index], fullName[index+1:]
+	}
+	return fullName, defaultNamespace
+}